1. Field of the Invention
The present invention relates to a cold cathode tube lighting device and a driving method and an integrated circuit to be employed in the cold cathode tube lighting device and more particularly to the cold cathode tube lighting device being suitably used when the cold cathode tube being used as a backlight of a liquid crystal display device is driven by applying voltages to input terminals on both ends of the cold cathode tube using separately-excited inverters and to the driving method and the integrated circuit to be used in the cold cathode tube lighting device.
The present application claims priority of Japanese Patent Application No. 2005-054698 filed on Feb. 28, 2005, which is hereby incorporated by reference.
2. Description of the Related Art
In recent years, a liquid crystal display device is used not only for monitors of personal computers, but also for various display devices such as a liquid crystal display television set. In the case of liquid crystal display television sets or a like in particular, upsizing of a liquid crystal panel progresses. As a result, backlights used in liquid crystal display devices are becoming larger in size and cold cathode tubes used in the backlights are also becoming longer. When the cold cathode tube as above is lit, in the case of shorter cold cathode tubes, its one input terminal is used as a low-voltage side and another input terminal is used as a high-voltage side and a driving pulse voltage is input to the high-voltage side. However, in the case of the longer cold cathode tubes or the cold cathode tubes having a small diameter, since impedance of the cold cathode tubes is made higher, when a driving pulse voltage is input from one input terminal (high-voltage side) of the cold cathode tubes, a display area in a region near the input terminal on the high-voltage side becomes brighter and the display area in a region near to the input terminal on the low-voltage side becomes darker, causing a luminance gradient (uneven lighting) to occur. To prevent the occurrence of the luminance gradient (uneven lighting), a both-side high-voltage driving method is employed in which the cold cathode tubes are made to light by applying driving pulse voltages which are 180° out of phase with each other to the input terminals on both ends of the cold cathode tubes. Moreover, in order to improve the efficiency of the backlight, even in the case where the cold cathode tubes are of “U”-shaped or “C”-shaped, the both-side high-voltage driving method is also used in some cases.
A conventional cold cathode tube lighting device of this type, as shown in FIG. 11, includes an oscillator 1, driving sections 2 and 3, transformers 4 and 5, and resonance capacitors 6 and 7. To output sides of the transformers 4 and 5 is connected a cold cathode tube 8. In the cold cathode tube lighting device, an oscillation frequency of the oscillator 1 is set at a frequency close to a resonant frequency of a resonant circuit made up of an inductance on each of transformer secondary sides 4b and 5b of the transformers 4 and 5 and by each of the resonance capacitors 6 and 7. High-frequency voltages having a frequency set by the oscillator 1 are generated and input to each of the transformer primary sides 4a and 5a of the transformers 4 and 5 and driving pulse voltages “e1” and “e2”, which are 180° out of phase with each other, output from the transformer secondary sides 4b and 5b of the transformers 4 and 5, respectively. Each of the driving pulse voltages “e1” and “e2” is applied to each of input terminals on both ends of the cold cathode tube 8 which lights the cold cathode tube 8.
Other conventional technologies of this type, besides the cold cathode tube lighting device as described above, are disclosed in following references. That is, a conventional piezoelectric transformer driving device disclosed in Patent Reference 1 (Japanese Patent Application Laid-open No. 2002-017090, Abstract, FIG. 1), as shown in FIG. 12, includes a power source 11, a driving circuit 12, a variable oscillating circuit 13, an oscillation controlling circuit 14, a piezoelectric transformer 15, a voltage detecting circuit 16, a current detecting circuit 17, a phase difference detecting circuit 18, and an active current detecting circuit 19. Between the piezoelectric transformer 15 and the current detecting circuit 17 is connected a cold cathode tube 20. In the vicinity of the cold cathode tube 20 is provided a reflecting plate 21 being grounded and floating capacitance Cx is formed between the cold cathode tube 20 and the reflecting plate 21. In the conventional piezoelectric transformer driving device, a tube current (a current output from the piezoelectric transformer 15) flowing through the cold cathode tube 20 is detected by the current detecting circuit 17 and a phase difference between output voltage and output current from the piezoelectric transformer 15 is detected by the phase difference detecting circuit 18. Based on the phase difference, an active current flowing through the cold cathode tube 20 is detected by the active current detecting circuit 19 and the piezoelectric transformer 15 is so controlled as to be driven via the oscillation controlling circuit 14, variable oscillating circuit 13, and driving circuit 12 so that the active current becomes a predetermined value.
A conventional piezoelectric transformer driving circuit disclosed in Patent Reference 2 (Japanese Patent Application Laid-open No. 2003-324962, Abstract, FIG. 1), as shown in FIG. 13, includes a piezoelectric transformer driving section 31, piezoelectric transformers 32 and 33, and a cold cathode tube 34 is connected to output sides of the piezoelectric transformer 32 and 33. The piezoelectric transformer driving circuit also has a current transformer 35, a resistor R, capacitors C1 and C2, a differential amplifying section 36, and a rectifying section 37. In the piezoelectric transformer driving circuit, a tube current flowing through a load (cold cathode tube 34) is detected by the current transformer 35 and an output from a resonant circuit formed by a secondary side component of the current transformer 35 and of each of capacitors C1 and C2 is fed back to the piezoelectric transformer driving section 31 through the differential amplifying section 36 and the rectifying section 37 so that an output from the piezoelectric transformer driving section 31 is controlled.
In an inverter circuit of a discharge lamp lighting device disclosed in Patent Reference 3 (Japanese Patent Application Laid-open No. 2003-168584, Abstract, FIG. 1), an output frequency is controlled according to a dimming ratio indicated by a dimming signal. A change in the output frequency causes a voltage applied to a discharge lamp to be changed. A filament voltage detecting circuit detects a voltage across a filament of the discharge lamp. A judging circuit, when judging that the discharge lamp operates abnormally at time of a rise in the output voltage in the filament voltage detecting circuit, stops operations of the inverter circuit. This enables an exact detection of such an abnormality as may occur at an end of a life of the discharge lamp.
In a discharge lamp lighting device disclosed in Patent Reference 4 (Japanese Patent Application Laid-open No. Hei 11-204277, Abstract, FIG. 1), a change in impedance of each filament of a plurality of discharge lamps is detected. When an abnormal change is detected in impedance of at least one filament at time of pre-heating, sufficient preheating power is supplied to remaining filaments and, after being preheated, a stable operation of the corresponding discharge lamp is started.
However, the conventional cold cathode tube lighting devices described above have the following problems. That is, luminance in a cold cathode tube is determined by a tube current flowing through the cold cathode tube. In the one-side high-voltage driving method in which a driving pulse voltage is input from an input terminal on one side of the cold cathode tube, in many cases, a current detecting circuit made up of a resistor or a like is provided on a low-voltage side where no driving pulse voltage is input and control is exerted to keep luminance in the cold cathode tube constant based on a current detected by the current detecting circuit. However, in the both-side high-voltage driving method using separately-excited inverters as shown in FIG. 11, driving pulse voltages “e1” and “e2” are applied to both input terminals of the cold cathode tube 8 and a current detecting circuit such as a resistor cannot be inserted, which presents a problem that detection of a tube current flowing through the cold cathode tube 8 is made difficult, causing keeping the luminance of the cold cathode tube 8 constant to be impossible. Moreover, in the case where the cold cathode tube 8 is driven by using the separately-excited inverters, as shown in FIG. 14, a current flowing through transformer secondary sides 4b and 5b of the transformers 4 and 5 contains a tube current flowing through the cold cathode tube 8 and a current flowing through the resonance capacitors 6 and 7, which presents a problem that, even if control is exerted so that current values on the transformer secondary sides 4b and 5b of the transformers 4 and 5 become constant, a ratio of the current flowing through the resonance capacitors 6 and 7 to the tube current flowing through the cold cathode tube 8 changes due to secular changes in characteristics of the cold cathode tube 8, also resulting in the luminance of the cold cathode tube 8 to deteriorate.
Moreover, the conventional piezoelectric transformer driving device disclosed in the Patent Reference 1 has a problem that, since a voltage output from its piezoelectric transformer 15 is high, as a component to which the high voltage is applied, the use of a high voltage tolerant component is required, which causes a rise in costs of the driving device. Furthermore, another problem is that, since a tube current is detected only on one side of the cold cathode tube 20, due to terminal-to-terminal variations of the piezoelectric transformer 15 and/or the cold cathode tube 20, exact detection of the tube current is impossible.
Also, the conventional piezoelectric driving device disclosed in the Patent Reference 2 has also a similar problem that, since voltages output from its piezoelectric transformers 32 and 33 are high, as a component to which the high voltages are applied, the use of a high voltage tolerant component is required, which causes a rise in costs of the driving device. Furthermore, another problem is that, since a tube current is detected only on one side of the cold cathode tube 34, due to terminal-to-terminal variations of the piezoelectric transformers 32 and 33 and/or the cold cathode tube 34, exact detection of the tube current is impossible.
In the discharge lamp lighting device disclosed in Patent Reference 3, though it is possible to exactly detect such an abnormality as may occur at an end of a life of the discharge lamp, it is impossible to keep its luminance constant.
In the discharge lamp lighting device disclosed in Patent Reference 4, though a stable operation of the discharge lamp other than the discharge lamps in which an abnormal change in impedance is detected is started, it is impossible to keep its luminance constant.